A reaction drive, also known as a pressure-jet and a tip-jet system, have been used successfully in the past to provide rotor power for helicopters. Reaction drive helicopters differ from conventional helicopters in that the rotor power is provided by the thrust of jets mounted at the blade-tips. This eliminates the mechanical transmission systems of conventional helicopters leading to a much lighter aircraft, requiring less energy to move. Reaction drive helicopters have a number of variants which, for the purposes of this invention, are considered to be divided into a first type in which air or gasses are directed through the blades and out a nozzle at the blade tip, and a second type in which a motor is positioned at the blade tip. The first type is typically differentiated on the basis of the air or gas temperature exiting through the jet nozzle at the tips of the helicopter blades. Usually these are labeled hot, warm or cold cycle tip-jet systems and are generated remotely from the blade tip. It is recognized that reaction drive helicopters are part of a larger group of related propulsion units that are generally termed reactive jet drive rotor systems. This larger group encompasses other helicopter rotor tip driven systems including the second type, in which motors such as turbojets, rockets, ramjets, pulse jets and other combustion engines attached to the blade tips have been used to provide rotor power for lifting and forward flight purposes. This invention is concerned with the first type of reaction drive helicopter.
In the field of aeronautics, circulation control is an approach used to modify an airfoil's aerodynamic forces using a specially shaped trailing edge instead of moving surfaces such as flaps. The main purpose of circulation control is to increase the lifting force of the airfoil at times when large lifting forces at low speeds are required, such as takeoff and landing. Circulation control airfoils take advantage of the Coanda effect which increases lift through the interaction of an air jet flowing through a slot in the trailing edge of the airfoil and a free air stream moving over the airfoil's upper surface as the airfoil moves through the air. A jet of air flows out of the slot and follows the curvature of a highly curved lower surface of the airfoil. The jet of air from the slot entrains the free air stream moving over the airfoil to create a laminar flow around the curvature, creating lift.
While circulation control systems work well on conventional airfoils, the use on a reaction drive helicopter is problematic. The air flow supplied to the rotor blades is employed for powering the rotors by being released through nozzles at the blade tips. Another problem using circulation control on rotor blades is flow separation. The flow over the Coanda surface typically separates at an angle between 120-125° from the slot. This separation can become periodic in nature with the separation point alternating between the two angles. This separation “flipping” can cause vibration and periodic lift forces.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
An object of this invention is to use circulation control on a reaction drive helicopter.
Another object is to minimize the unsteady lift phenomenon that occurs due to separation over the Coanda surface.